Uninterruptible power supply apparatus

ABSTRACT

A control device of an uninterruptible power supply apparatus turns on a first switch and a second switch and turns off a third switch in an inverter power feed mode, turns on the first and third switches and turns off the second switch in a bypass power feed mode, and executes a lap power feed mode of turning off the first switch and turning on the second and third switches in a switching period in which one mode of the inverter power feed mode and the bypass power feed mode is switched to the other mode.

TECHNICAL FIELD

The present invention relates to an uninterruptible power supply apparatus, and more particularly to an uninterruptible power supply apparatus having an inverter power feed mode in which AC power is supplied from an inverter to a load, a bypass power feed mode in which AC power is supplied from a bypass AC power supply to the load, and a lap power feed mode in which AC power is supplied from both of the inverter and the bypass AC power supply to the load.

BACKGROUND ART

For example, WO2017/017719 (PTL 1) discloses an uninterruptible power supply apparatus having an inverter power feed mode, a bypass power feed mode, and a lap power feed mode. This uninterruptible power supply apparatus includes a converter configured to convert AC voltage supplied from a commercial AC power supply to DC voltage, a capacitor configured to smooth DC output voltage from the converter, an inverter configured to convert terminal-to-terminal voltage of the capacitor to AC voltage, a first switch having one terminal receiving AC output voltage of the inverter and the other terminal connected to a load, and a second switch having one terminal receiving AC voltage supplied from a bypass AC power supply and the other terminal connected to the load.

In the inverter power feed mode, the first switch is turned on and the second switch is turned off. In the bypass power feed mode, the second switch is turned on and the first switch is turned off. In the lap power feed mode, both the first and second switches are turned on. In the lap power feed mode, a switching period of switching between the inverter power feed mode and the bypass power feed mode is executed.

CITATION LIST Patent Literature

PTL 1: WO2017/017719

SUMMARY OF INVENTION Technical Problem

Unfortunately, in the conventional uninterruptible power supply apparatus, the commercial AC power supply and the bypass AC power supply each include a three phase AC power supply star-connected to a neutral point, and when both of the neutral points of the commercial AC power supply and the bypass AC power supply are grounded, circulating current may flow from one AC power supply of the commercial AC power supply and the bypass AC power supply to the other AC power supply through the capacitor in the lap power feed mode (see FIG. 6, FIG. 7). If a large circulating current flows, overcurrent is detected, or overvoltage of the capacitor is detected, so that the operation of the uninterruptible power supply apparatus is stopped, and the operation of the load is stopped.

A main object of the present invention is therefore to provide an uninterruptible power supply apparatus capable of preventing circulating current from flowing even when the neutral points of the first and second AC power supplies are grounded.

Solution to Problem

An uninterruptible power supply apparatus according to the present invention includes a forward converter, a capacitor, a reverse converter, a bidirectional chopper, a first switch, a second switch, a third switch, a first control unit, and a second control unit. The forward converter converts three phase AC voltage to DC voltage. The capacitor smooths DC output voltage of the forward converter. The reverse converter converts terminal-to-terminal voltage of the capacitor to three phase AC voltage. The bidirectional chopper exchanges DC power between the capacitor and a power storage device.

The first switch is disposed corresponding to each phase of three phase AC voltage supplied from a first AC power supply and has one terminal receiving AC voltage of a corresponding phase and another terminal connected to the forward converter. The second switch is disposed corresponding to each phase of three phase AC voltage supplied from the reverse converter and has one terminal receiving AC voltage of a corresponding phase and another terminal connected to a load. The third switch is disposed corresponding to each phase of three phase AC voltage supplied from a second AC power supply and has one terminal receiving AC voltage of a corresponding phase and another terminal connected to the load.

The first control unit is configured to turn on the first and second switches and turn off the third switch in a first mode of supplying three phase AC voltage from the reverse converter to the load, to turn on the first and third switches and turn off the second switch in a second mode of supplying three phase AC voltage from the second AC power supply to the load, and to turn off the first switch and turn on the second and third switches and execute a third mode of supplying three phase AC voltage from both of the reverse converter and the second AC power supply to the load in a switching period in which one mode of the first and second modes is switched to another mode.

The second control unit is configured to control the reverse converter in synchronization with three phase AC voltage supplied from the second AC power supply, to control the forward converter such that terminal-to-terminal voltage of the capacitor becomes a first reference voltage and control the bidirectional chopper such that terminal-to-terminal voltage of the power storage device becomes a second reference voltage, in the first and second modes, and to stop operation of the forward converter and control the bidirectional chopper such that terminal-to-terminal voltage of the capacitor becomes a third reference voltage, in the switching period.

Advantageous Effects of Invention

In the uninterruptible power supply apparatus according to the present invention, in the switching period in which the first mode and the second mode are switched, the first switch is turned off to electrically isolate the first AC power supply from the forward converter. This can prevent circulating current from flowing even when both of the neutral points of the first and second AC power supplies are grounded.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit block diagram showing a configuration of an uninterruptible power supply apparatus according to a first embodiment.

FIG. 2 is a circuit block diagram showing a configuration of a converter and an inverter shown in FIG. 1.

FIG. 3 is an equivalent circuit diagram showing a configuration of a commercial AC power supply shown in FIG. 1.

FIG. 4 is an equivalent circuit diagram showing a configuration of a bypass AC power supply shown in FIG. 1.

FIG. 5 is a diagram showing the relation between three phase AC voltage of the commercial AC power supply shown in FIG. 3 and three phase AC voltage of the bypass AC power supply.

FIG. 6 is a circuit block diagram for explaining the effect of the present invention.

FIG. 7 is another circuit block diagram for explaining the effect of the present invention.

FIG. 8 is a block diagram showing the main part of a control device shown in FIG. 1.

FIG. 9 is a circuit block diagram showing a configuration of a control unit 16 shown in FIG. 8.

FIG. 10 is a circuit block diagram showing a configuration of a bidirectional chopper shown in FIG. 1.

FIG. 11 is a time chart showing the operation of the control device shown in FIG. 8 to FIG. 10.

FIG. 12 is another time chart showing the operation of the control device shown in FIG. 8 to FIG. 10.

FIG. 13 is a circuit block diagram showing a configuration of an uninterruptible power supply apparatus according to a second embodiment.

FIG. 14 is a time chart showing the operation of a control unit 40 shown in FIG. 13.

FIG. 15 is another time chart showing the operation of control unit 40 shown in FIG. 13.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is a circuit block diagram showing a configuration of an uninterruptible power supply apparatus according to a first embodiment of the present invention. In FIG. 1, this uninterruptible power supply apparatus includes switches S1 to S9, capacitors C1 to C6 and Cd, reactors L1 to L6, current detectors CT1 to CT6, a converter 1, a DC positive bus Lp, a DC negative bus Ln, a bidirectional chopper 2, an inverter 3, an operation unit 4, and a control device 5.

This uninterruptible power supply apparatus receives three phase AC power with a commercial frequency from a commercial AC power supply 6 and a bypass AC power supply 7 and supplies three phase AC power with a commercial frequency to a load 8. Commercial AC power supply 6 outputs three phase AC voltages Vu1, Vv1, and Vw1 to AC output terminals 6 a to 6 c, respectively. A neutral point terminal 6 d of commercial AC power supply 6 receives ground voltage GND.

Instantaneous values of three phase AC voltages Vu1, Vv1, and Vw1 are detected by control device 5. Control device 5 detects whether a power failure of commercial AC power supply 6 has occurred, based on AC output voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6.

Bypass AC power supply 7 outputs three phase AC voltages Vu2, Vv2, and Vw2 to AC output terminals 7 a to 7 c, respectively. A neutral point terminal 7 d of bypass AC power supply 7 receives ground voltage GND. Instantaneous values of three phase AC voltages Vu2, Vv2, and Vw2 are detected by control device 5. AC input terminals 8 a to 8 c of load 8 receive three phase AC voltage from the uninterruptible power supply apparatus. Load 8 is driven by three phase AC power supplied from the uninterruptible power supply apparatus.

One terminal of each of switches S1 to S3 is connected to the corresponding one of AC output terminals 6 a to 6 c of commercial AC power supply 6. Capacitor C1 to C3 each have one electrode connected to the other terminal of the corresponding one of switches S1 to S3 and have the other electrodes connected to each other. Reactors L1 to L3 each have one terminal connected to the other terminal of the corresponding one of switches S1 to S3 and have the other terminals connected to three input nodes of converter 1.

Switches S1 to S6 are controlled by control device 5. In an inverter power feed mode (first power feed mode) in which three phase AC power generated by inverter 3 is supplied to load 8, control device 5 turns on switches S1 to S6 and turns off switches S7 to S9. In a bypass power feed mode (second power feed mode) in which three phase AC power from bypass AC power supply 7 is supplied to load 8, control device 5 turns on switches S1 to S3 and S7 to S9 and turns off switches S4 to S6. In a switching period in which one power feed mode of the inverter power feed mode and the bypass power feed mode is switched to the other power feed mode, the control device 5 turns off switches S1 to S3 to electrically isolate commercial AC power supply 6 from converter 1.

Capacitors C1 to C3 and reactors L1 to L3 constitute an AC filter F1. AC filter F1 is a low pass filter, allows AC current with a commercial frequency to flow from commercial AC power supply 6 to converter 1, and prevents a signal with a switching frequency from flowing from converter 1 to commercial AC power supply 6. Current detectors CT1 to CT3 detect AC currents I1 to I3 flowing through reactors L1 to L3, respectively, and apply a signal indicating a detected value to control device 5.

The positive-side output node of converter 1 is connected to the positive-side input node of inverter 3 through DC positive bus Lp. The negative-side output node of converter 1 is connected to the negative-side input node of inverter 3 through DC negative bus Ln. Capacitor Cd is connected between buses Lp and Ln and smooths DC voltage VDC between buses Lp and Ln. An instantaneous value of DC voltage VDC is detected by control device 5.

Converter 1 is controlled by control device 5 and converts three phase AC power from commercial AC power supply 6 to DC power when three phase AC power is supplied normally from commercial AC power supply 6 (in a sound state of commercial AC power supply 6). DC power generated by converter 1 is supplied to bidirectional chopper 2 and inverter 3 through buses Lp and Ln.

In a sound state of commercial AC power supply 6, control device 5 controls converter 1 such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr1 (first reference voltage), based on AC output voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6, AC currents I1 to I3, and terminal-to-terminal voltage VDC of capacitor Cd. When supply of three phase AC power from commercial AC power supply 6 is stopped (at the time of a power failure of commercial AC power supply 6), control device 5 stops the operation of converter 1.

In a switching period in which one power feed mode of the inverter power feed mode and the bypass power feed mode is switched to the other power feed mode, control device 5 stops the operation of converter 1. AC filter F1 and converter 1 constitute a forward converter that converts three phase AC power from commercial AC power supply 6 to DC power.

Bidirectional chopper 2 is controlled by control device 5, stores DC power generated by converter 1 into battery 9 in a sound state of commercial AC power supply 6, and supplies DC power in battery 9 to inverter 3 through buses Lp and Ln in response to occurrence of a power failure of commercial AC power supply 6. An instantaneous value of terminal-to-terminal voltage VB of battery 9 is detected by control device 5.

Control device 5 controls bidirectional chopper 2, based on terminal-to-terminal voltage VDC of capacitor Cd and terminal-to-terminal voltage VB of battery 9. In a sound state of commercial AC power supply 6, control device 5 controls bidirectional chopper 2 such that terminal-to-terminal voltage VB of battery 9 becomes reference voltage VBr (second reference voltage). At the time of a power failure of commercial AC power supply 6, control device 5 controls bidirectional chopper 2 such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2 (third reference voltage).

In the switching period in which one power feed mode of the inverter power feed mode and the bypass power feed mode is switched to the other power feed mode, control device 5 controls bidirectional chopper 2 such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2. It is noted that VBr<VDC2 r<VDC1. VDCr2 is a voltage slightly lower than VDCr1.

Inverter 3 is controlled by control device 5 and converts DC power supplied from converter 1 and bidirectional chopper 2 to three phase AC power with a commercial frequency. Each of three output nodes of inverter 3 is connected to one terminal of the corresponding one of reactors L4 to L6. The other terminal of each of reactors L4 to L6 is connected to one terminal of the corresponding one of switches S4 to S6, and the other terminals of switches S4 to S6 are respectively connected to three AC input terminals 8 a to 8 c of load 8. One electrode of each of capacitors C4 to C6 is connected to the other terminal of the corresponding one of reactors L4 to L6, and the other electrodes of capacitors C4 to C6 are connected together to the other electrodes of capacitors C1 to C3.

Capacitors C4 to C6 and reactors L4 to L6 constitute an AC filter F2. AC filter F2 is a low pass filter, allows AC current with a commercial frequency to flow from inverter 3 to load 8, and prevents a signal with a switching frequency from flowing from inverter 3 to load 8. In other words, AC filter F2 converts three phase rectangular wave voltage output from inverter 3 to sinusoidal three phase AC voltages Va, Vb, and Vc.

Instantaneous values of three phase AC voltages Va to Vc are detected by control device 5. Current detectors CT4 to CT6 detect AC currents I4 to I6 flowing through reactors L4 to L6, respectively, and apply a signal indicating a detected value to control device 5.

Control device 5 controls inverter 3 such that AC output voltages Va to Vc become AC output voltages Vu2, Vv2, and Vw2, respectively, based on AC output voltages Va to Vc of inverter 3, AC output voltages Vu2, Vv2, and Vw2 of bypass AC power supply 7, and AC currents I4 to I6.

Switches S7 to S9 each have one terminal connected to the corresponding one of AC output terminals 7 a to 7 c of bypass AC power supply 7 and have the other terminals respectively connected to AC input terminals 8 a to 8 c of load 8. Switches S7 to S9 are controlled by control device 5 and are turned off in the inverter power feed mode and turned on in the bypass power feed mode and the lap power feed mode.

Operation unit 4 (selector) includes a plurality of buttons operated by a user of the uninterruptible power supply apparatus and an image display unit presenting a variety of information. The user can operate operation unit 4 to power on and off the uninterruptible power supply apparatus and select one mode of the automatic operation mode, the bypass power feed mode, and the inverter power feed mode.

Control device 5 controls the entire uninterruptible power supply apparatus based on a signal from operation unit 4, AC output voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6, AC input currents I1 to I3, terminal-to-terminal voltage VDC of capacitor Cd, terminal-to-terminal voltage VB of battery 9, AC output currents I4 to I6, AC output voltages Va to Vc, and AC output voltages Vu2, Vv2, and Vw2 of bypass AC power supply 7, and the like.

The operation of this uninterruptible power supply apparatus will now be described briefly. When the automatic operation mode is selected using operation unit 4 in a sound state of commercial AC power supply 6, switches S1 to S3 are turned on so that commercial AC power supply 6 is connected to converter 1 through AC filter F1, switches S4 to S6 are turned on so that inverter 3 is connected to load 8 through AC filter F2, and switches S7 to S9 are turned off so that bypass AC power supply 7 is electrically isolated from load 8.

Converter 1 is controlled such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr1, bidirectional chopper 2 is controlled such that terminal-to-terminal voltage VB of battery 9 becomes reference voltage VBr, and inverter 3 is controlled such that AC output voltages Va to Vc become AC output voltages Vu2, Vv2, and Vw2, respectively, of bypass AC power supply 7. AC output voltages Va to Vc are thus supplied to load 8 through switches S4 to S6 to drive load 8.

When a power failure of commercial AC power supply 6 occurs, switches S1 to S3 are turned off, the operation of converter 1 is stopped, bidirectional chopper 2 is controlled such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2, and inverter 3 is controlled such that AC output voltages Va to Vc become AC output voltages Vu2, Vv2, and Vw2, respectively, of bypass AC power supply 7.

When DC power of battery 9 is consumed and terminal-to-terminal voltage VB of battery 9 reaches a lower limit value, the operation of bidirectional chopper 2 and inverter 3 is stopped. Thus, even when a power failure of commercial AC power supply 6 occurs, the operation of load 8 can be continued for a period until the terminal-to-terminal voltage VB of battery 9 reaches the lower limit value.

When the inverter power feed mode is selected using operation unit 4 in a sound state of commercial AC power supply 6, switches S1 to S6 are turned on and switches S7 to S9 are turned off, converter 1 is controlled such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr1, and bidirectional chopper 2 is controlled such that terminal-to-terminal voltage VB of battery 9 becomes reference voltage VBr, in the same manner as in the automatic operation mode. Inverter 3 is controlled such that AC output voltages Va to Vc become AC output voltages Vu2, Vv2, and Vw2, respectively, of bypass AC power supply 7.

When the bypass power feed mode is selected using operation unit 4 in the inverter power feed mode, switches S1 to S3 are turned off, the operation of converter 1 is stopped, and bidirectional chopper 2 is controlled such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2. Subsequently, the lap power feed mode is performed for a predetermined period of time, switches S4 to S9 are turned on, and three phase AC power is supplied from both of inverter 3 and bypass AC power supply 7 to load 8. At this point of time, since switches S1 to S3 are off, circulating current does not flow through the uninterruptible power supply apparatus.

When the lap power feed mode ends, switches S4 to S6 are turned off and only switches S7 to S9 are turned on. Subsequently, switches S1 to S3 are turned on, converter 1 is controlled so that terminal-to-terminal voltage VDC of capacitor Cd is raised to reference voltage VDCr1, and the switching from the inverter power feed mode to the bypass power feed mode is completed. When the operation of converter 1 is resumed, DC power generated by converter 1 is stored into battery 9 through bidirectional chopper 2.

In the bypass power feed mode, three phase AC power is supplied from bypass AC power supply 7 to load 8 through switches S7 to S9 to drive load 8. In the bypass power feed mode, converter 1, bidirectional chopper 2, and inverter 3 are operated in preparation for the next time the inverter power feed mode is selected, and for example, battery 9 is charged. Alternatively, in the bypass power feed mode, switches S1 to S3 are turned off, and for example, repair or routine check of converter 1, bidirectional chopper 2, inverter 3, battery 9, etc. is performed.

When the inverter power feed mode is selected using operation unit 4 in the bypass power feed mode, switches S1 to S3 are turned off the operation of converter 1 is stopped, and bidirectional chopper 2 is controlled such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2. Subsequently, the lap power feed mode is performed for a predetermined period of time, switches S4 to S9 are turned on, and three phase AC power is supplied from both of inverter 3 and bypass AC power supply 7 to load 8. At this point of time, since switches S1 to S3 are off, circulating current does not flow through the uninterruptible power supply apparatus.

When the lap power feed mode ends, switches S7 to S9 are turned off, switches S1 to S6 are turned on, converter 1 raises terminal-to-terminal voltage VDC of capacitor Cd to reference voltage VDCr1, and the switching from the bypass power feed mode to the inverter power feed mode is completed. When converter 1 is operated, DC power generated by converter 1 is stored into battery 9 through bidirectional chopper 2.

Circulating current flowing through such an uninterruptible power supply apparatus will now be described. FIG. 2 is a circuit diagram showing a configuration of converter 1 and inverter 3. In FIG. 2, converter 1 includes IGBTs (Insulated Gate Bipolar Transistors) Q1 to Q6 and diodes D1 to D6. The IGBTs constitute a switching element. The collectors of IGBTs Q1 to Q3 are connected together to DC positive bus Lp, and the emitters thereof are respectively connected to input nodes 1 a, 1 b, and 1 c.

Input nodes 1 a, 1 b, and 1 c are respectively connected to the other terminals of reactors L1 to L3 (FIG. 1). The collectors of IGBTs Q4 to Q6 are respectively connected to input nodes 1 a, 1 b, and 1 c, and the emitters thereof are connected together to DC negative bus Ln. Diodes D1 to D6 are respectively connected in anti-parallel with IGBTs Q1 to Q6.

IGBTs Q1 and Q4 are respectively controlled by gate signals A1 and B1, IGBTs Q2 and Q5 are respectively controlled by gate signals A2 and B2, and IGBTs Q3 and Q6 are respectively controlled by gate signals A3 and B3. Gate signals B1, B2, and B3 are inversion signals of gate signals A1, A2, and A3, respectively.

IGBTs Q1 to Q3 turn on when gate signals A1, A2, and A3 are brought to “H” level, respectively, and turn off when gate signals A1, A2, and A3 are brought to “L” level, respectively. IGBTs Q4 to Q6 turn on when gate signals B1, B2, and B3 are brought to “H” level, respectively, and turn off when gate signals B1, B2, and B3 are brought to “L” level, respectively.

Each of gate signals A1, B1, A2, B2, A3, and B3 is a pulse signal train and a PWM (Pulse Width Modulation) signal. The phase of gate signal A1, B1, the phase of gate signal A2, B2, and the phase of gate signal A3, B3 are basically shifted from each other by 120 degrees. Gate signals A1, B1, A2, B2, A3, and B3 are generated by control device 5.

For example, when the level of AC input voltage Vu1 is higher than the level of AC input voltage Vv1, IGBTs Q1 and Q5 are turned on, and current flows from input node 1 a to input node 1 b through IGBT Q1, DC positive bus Lp, capacitor Cd, DC negative bus Ln, and IGBT Q5 to charge capacitor Cd.

Conversely, when the level of AC input voltage Vv1 is higher than the level of AC input voltage Vu1, IGBTs Q2 and Q4 are turned on, and current flows from input node 1 b to input node 1 a through IGBT Q2, DC positive bus Lp, capacitor Cd, DC negative bus Ln, and IGBT Q4 to charge capacitor Cd. This is the same in other cases.

Each of IGBTs Q1 to Q6 is turned on and off at a predetermined timing by gate signals A1, B1, A2, B2, A3, and B3, and the ON time of each of IGBTs Q1 to Q6 is adjusted, whereby three phase AC voltage applied to input nodes 6 a to 6 c can be converted to DC voltage VDC (terminal-to-terminal voltage of capacitor Cd).

Inverter 3 includes IGBTs Q11 to Q16 and diodes D11 to D16. The IGBTs constitute a switching element. The collectors of IGBTs Q11 to Q13 are connected together to DC positive bus Lp, and the emitters thereof are respectively connected to output nodes 3 a, 3 b, and 3 c. Each of output nodes 3 a, 3 b, and 3 c is connected to one terminal of the corresponding one of reactors L4 to L6 (FIG. 1). The collectors of IGBTs Q14 to Q16 are respectively connected to output nodes 3 a, 3 b, and 3 c, and the emitters thereof are connected together to DC negative bus Ln. Diodes D11 to D16 are respectively connected in anti-parallel with IGBTs Q11 to Q16.

IGBTs Q11 and Q14 are respectively controlled by gate signals X1 and Y1, IGBTs Q12 and Q15 are respectively controlled by gate signals X2 and Y2, and IGBTs Q13 and Q16 are respectively controlled by gate signals X3 and Y3. Gate signals Y1, Y2, and Y3 are inversion signals of gate signals X1, X2, and X3, respectively.

IGBTs Q11 to Q13 turn on when gate signals X1, X2, and X3 are brought to “H” level, respectively, and turn off when gate signals X1, X2, and X3 are brought to “L” level, respectively. IGBTs Q14 to Q16 turn on when gate signals Y1, Y2, and Y3 are brought to “H” level, respectively, and turn off when gate signals Y1, Y2, and Y3 are brought to “L” level, respectively.

Each of gate signals X1, Y2, X3, Y1, X2, and Y3 is a pulse signal train and a PWM signal. The phase of gate signal X1, Y1, the phase of gate signal X2, Y2, and the phase of gate signal X3, Y3 are basically shifted from each other by 120 degrees. Gate signals X1, Y1, X2, Y2, X3, and Y3 are generated by control device 5.

For example, when IGBTs Q11 and Q15 turn on, DC positive bus Lp is connected to output node 3 a through IGBT Q11, output node 3 b is connected to DC negative bus Ln through IGBT Q15, and a positive voltage is output between output nodes 3 a and 3 b.

When IGBTs Q12 and Q14 turn on, DC positive bus Lp is connected to output node 3 b through IGBT Q12, output node 3 a is connected to DC negative bus Ln through IGBT Q14, and a negative voltage is output between output nodes 3 a and 3 b.

Each of IGBTs Q11 to Q16 is turned on and off at a predetermined timing by gate signals X1, Y1, X2, Y2, X3, and Y3, and the ON time of each of IGBTs Q11 to Q16 is adjusted, whereby DC voltage VDC between buses Lp and Ln can be converted to three phase AC voltages Va, Vb, and Vc.

FIG. 3 is an equivalent circuit diagram showing a configuration of commercial AC power supply 6. In FIG. 3, commercial AC power supply 6 includes three phase AC power supplies 6U, 6V, and 6W star-connected (Y-connected) to neutral point terminal 6 d. AC power supply 6U is connected between AC output terminal 6 a and neutral point terminal 6 d and outputs AC voltage Vu1 to AC output terminal 6 a. AC power supply 6V is connected between AC output terminal 6 b and neutral point terminal 6 d and outputs AC voltage Vv1 to AC output terminal 6 b. AC power supply 6W is connected between AC output terminal 6 c and neutral point terminal 6 d and outputs AC voltage Vw1 to AC output terminal 6 c.

Each of AC voltages Vu1, Vv1, and Vw1 changes sinusoidally at a commercial frequency (for example, 60 Hz). The peak values (42 times the effective value) of AC voltages Vu1, Vv1, and Vw1 are the same, and the phases thereof are shifted from each other by 120 degrees. AC power supplies 6U, 6V, and 6W correspond to, for example, three phase windings at the last stage included in a three phase transformer at the last stage of commercial AC power supply 6.

FIG. 4 is an equivalent circuit diagram showing a configuration of bypass AC power supply 7. In FIG. 4, bypass AC power supply 7 includes three phase AC power supplies 7U, 7V, and 7W star-connected to neutral point terminal 7 d. AC power supply 7U is connected between AC output terminal 7 a and neutral point terminal 7 d and outputs AC voltage Vu2 to AC output terminal 7 a. AC power supply 7V is connected between AC output terminal 7 b and neutral point terminal 7 d and outputs AC voltage Vv2 to AC output terminal 7 b. AC power supply 7W is connected between AC output terminal 7 c and neutral point terminal 7 d and outputs AC voltage Vw2 to AC output terminal 7 c.

Each of AC voltages Vu2, Vv2, and Vw2 changes sinusoidally at a commercial frequency. The peak values of AC voltages Vu2, Vv2, and Vw2 are the same, and the phases thereof are shifted from each other by 120 degrees. AC power supplies 7U, 7V, and 7W correspond to, for example, a three phase coil of a self-generator.

In the inverter power feed mode and the bypass power feed mode, the phases (and peak values) of AC voltages Vu2, Vv2, and Vw2 of bypass AC power supply 7 match the phases (and peak values) of AC voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6, respectively, and switches S7 to S9 or switches S4 to S6 are off. In this state, no circulating current flows through the uninterruptible power supply apparatus.

However, in the lap power feed mode, when switches S7 to S9 or switches S4 to S6 turn on, load current of bypass AC power supply 7 significantly fluctuates, and the phases and peak values of AC voltages Vu2, Vv2, and Vw2 fluctuate. AC voltages Vu2, Vv2, and Vw2 then do not match AC voltages Vu1, Vv1, and Vw1, respectively.

FIGS. 5(A) to 5(C) are diagrams showing the relation between AC voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6 and AC voltages Vu2, Vv2, and Vw2 of bypass AC power supply 7. Each of AC voltages Vu1, Vv1, Vw1, Vu2, Vv2, and Vw2 is illustrated by a vector. AC voltages Vu1, Vv1, and Vw1 are out of phase by 120 degrees, and AC voltages Vu2, Vv2, and Vw2 are out of phase by 120 degrees. FIG. 5(A) shows a case where the phases of AC voltages Vu2, Vv2, and Vw2 match the phases of AC voltages Vu1, Vv1, and Vw1, respectively.

FIG. 5(B) shows a case where the phases of AC voltages Vu2, Vv2, and Vw2 lag behind the phases of AC voltages Vu1, Vv1, and Vw1, respectively, by 60 degrees. For example, AC voltage Vu1 and AC voltage Vw2 are out of phase by 180 degrees. When AC voltage Vu1 is a positive peak value and AC voltage Vw2 is a negative peak value, voltage ΔV12=Vu1−Vw2 that is the difference between AC voltage Vu1 and AC voltage Vw2 is the sum of peak values of AC voltages Vu1 and Vw2. Conversely, when AC voltage Vu1 is a negative peak value and AC voltage Vw2 is a positive peak value, voltage ΔV21=Vw2−Vu1 that is the difference between AC voltage Vw2 and AC voltage Vu1 is the sum of peak values of AC voltages Vu1 and Vw2.

FIG. 5(C) shows a case where the phases of AC voltages Vu2, Vv2, and Vw2 are ahead of the phases of AC voltages Vu1, Vv1, and Vv1, respectively, by 60 degrees. For example, AC voltage Vu1 and AC voltage Vv2 are out of phase by 180 degrees. When AC voltage Vu1 is a positive peak value and AC voltage Vv2 is a negative peak value, voltage ΔV12=Vu1−Vv2 that is the difference between AC voltage Vu1 and AC voltage Vv2 is the sum of peak values of AC voltages Vu1 and Vv2. Conversely, when AC voltage Vu1 is a negative peak value and AC voltage Vv2 is a positive peak value, voltage ΔV21=Vv2−Vu1 that is the difference between AC voltage Vv2 and AC voltage Vu1 is the sum of peak values of AC voltages Vu1 and Vv2.

If in the lap power feed mode, switches S1 to S3 are turned on and terminal-to-terminal voltage VDC of capacitor Cd is smaller than the sum of peak values of AC voltages Vu1, Vv1, and Vw1 and peak values of AC voltages Vu2, Vv2, and Vw2, the following problem arises. For example, as shown in FIG. 5(B), when AC voltages Vu1 and Vw2 are out of phase 180 degrees and voltage ΔV12=Vu1−Vw2 that is the difference between AC voltages Vu1 and Vw2 is the sum of peak values of AC voltages Vu1 and Vw2, circulating current IC flows through the path shown in FIG. 6.

That is, circulating current IC flows through a path from one terminal (output terminal 6 a) of AC power supply 6U to the other terminal of AC power supply 6U through input node 1 a of converter 1, diode D1 (FIG. 2), DC positive bus Lp, capacitor Cd, DC negative bus Ln, diode D16 (FIG. 2), output node 3 c of inverter 3, AC power supply 7W, neutral point terminal 7 d, the line of ground voltage GND, and neutral point terminal 6 d. In FIG. 6, for the sake of simplicity of the drawing and the description, filters F1, F2, switches S1 to S9 turned on, and the like are not illustrated.

Conversely, when voltage ΔV21=Vw2−Vu1 that is the difference between AC voltages Vw2 and Vu1 is the sum of peak values of AC voltages Vu1 and Vw2, circulating current IC flows through the path shown in FIG. 7. That is, circulating current IC flows through a path from one terminal (output terminal 7 c) of AC power supply 7W to the other terminal of AC power supply 7U through output node 3 c of inverter 3, diode D13 (FIG. 2), DC positive bus Lp, capacitor Cd, DC negative bus Ln, diode D4 (FIG. 2), input node 1 a of converter 1, AC power supply 6U, neutral point terminal 6 d, the line of ground voltage GND, and neutral point terminal 7 d.

When circulating current IC flows, circulating current IC charges capacitor Cd, terminal-to-terminal voltage VDC of capacitor Cd may exceed upper limit value VDCH, and control device 5 may determine that abnormality has occurred, so that the operation of the uninterruptible power supply apparatus may be stopped and the operation of load 8 may be stopped. The detected values of current detectors CT1 to CT6 may exceed upper limit value IH, and the control device 5 may determine that abnormality has occurred, so that the uninterruptible power supply apparatus may be stopped and the operation of load 8 may be stopped.

Then, in the present first embodiment, in the switching period in which one power feed mode of the inverter power feed mode and the bypass power feed mode is switched to the other power feed mode, switches S1 to S3 are turned off to prevent circulating current IC from flowing through the uninterruptible power supply apparatus. The lap power feed mode is performed in the switching period.

In the present first embodiment, terminal-to-terminal voltage VDC of capacitor Cd is set to reference voltages VDCr1, VDCr2 lower than the voltage of the sum of peak values of AC voltages Vu1, Vv1, Vw1 and peak values of AC voltages Vu2, Vv2, Vw2 to reduce power consumption and improve the efficiency.

When bypass AC power supply 7 is stable, AC output voltages Vu2, Vv2, and Vw2 of bypass AC power supply 7 match AC output voltages Vu1, Vv1, and Vw1 of commercial AC power supply 6, and therefore the voltage of the sum of peak values of AC voltages Vu1, Vv1, and Vw1 and peak values of AC voltages Vu2, Vv2, and Vw2 is equal to the voltage twice the peak values of AC voltages Vu1, Vv1, and Vw1. The peak values of AC voltages Vu1, Vv1, and Vw1 are the same value.

For example, the effective value of AC voltage Vu1 is 277 V and the peak value thereof is 392 V. The voltage twice the peak value of AC voltage Vu1 is 784 V. Reference voltage VDCr1 is set to 750 V lower than 784 V. Reference voltage VDCr2 is set to 730 V slightly lower than reference voltage VDCr1. Reference voltage VDCr1 is set to a value lower than upper limit value VDCH (for example, 1000 V) of terminal-to-terminal voltage VDC of capacitor Cd.

A method of controlling converter 1 and switches S1 to S9 will now be described. FIG. 8 is a block diagram showing a configuration of a part of control device 5 that is related to control of converter 1 and switches S1 to S9. In FIG. 8, control device 5 includes a signal generating circuit 11, a timer 12, a power failure detector 13, and control units 14 to 16.

Operation unit 4 (FIG. 1) brings mode select signal MS to “L” level when the user of the uninterruptible power supply apparatus selects the inverter power feed mode, and brings mode select signal MS to “H” level when the user selects the bypass power feed mode. Signal generating circuit 11 raises switch command signal PC to “H” level for a predetermined period of time, in response to each of the rising edge and the falling edge of mode select signal MS from operation unit 4.

Timer 12 successively measures first time T1, second time T2, and third time T3, in response to the rising edge of switch command signal PC. Timer 12 brings switch signal ϕC to “H” level that is the active level from the rising edge of switch command signal PC to third time T3. Further, timer 12 brings overlap command signal ϕOL to “H” level that is the active level from first time T1 to second time T2.

Power failure detector 13 determines whether a power failure has occurred, based on three phase AC voltages Vu1, Vv1, and Vw1 supplied from commercial AC power supply 6, and outputs power failure detection signal ϕF indicating the determination result. For example, power failure detector 13 determines that commercial AC power supply 6 is sound when the levels of AC voltages Vu1, Vv1, and Vw1 are higher than a lower limit value, and determines that a power failure has occurred when the levels of AC voltages Vu1, Vv1, and Vw1 become lower than a lower limit value. When commercial AC power supply 6 is sound, power failure detection signal ϕF is brought to “L” level that is the inactive level. When a power failure has occurred, power failure detection signal ϕF is brought to “H” level that is the active level.

Control unit 14 controls switches S4 to S9 in accordance with mode select signal MS and overlap command signal ϕOL. When both of mode select signal MS and overlap command signal ϕOL are “L” level, control unit 14 turns on switches S4 to S6 and turns off switches S7 to S9.

When overlap command signal ϕOL is “H” level, control unit 14 turns on switches S4 to S9. When mode select signal MS is “H” level and overlap command signal ϕOL is “L” level, control unit 14 turns on switches S7 to S9 and turns off switches S4 to S6.

Control unit 15 turns off switches S1 to S3 when at least one of switch signal ϕC and power failure detection signal ϕF is brought to “H” level that is the active level, and turns on switches S1 to S3 when both of switch signal ϕC and power failure detection signal ϕF are “L” level that is the inactive level.

When both of switch signal ϕC and power failure detection signal ϕF are “L” level that is the inactive level, control unit 16 operates based on AC input voltages Vu1, Vv1, and Vw1, three phase input currents I1 to I3, and DC voltage VDC and controls converter 1 such that terminal-to-terminal voltage VDC of capacitor Cd matches reference voltage VDCr1 (or VDCr2). Control unit 16 stops the operation of converter 1 when at least one of switch signal ϕC and power failure detection signal ϕF is brought to “H” level that is the active level.

FIG. 9 is a circuit block diagram showing a configuration of control unit 16. In FIG. 9, control unit 16 includes voltage detectors 20 and 28, a reference voltage generating circuit 21, subtracters 22 and 26A to 26C, a DC voltage control circuit 23, a sine wave generating circuit 24, multipliers 25A to 25C, a current control circuit 27, adders 29A to 29C, a PWM circuit 30, an OR gate 31, and a gate circuit 32.

Voltage detector 20 detects terminal-to-terminal voltage VDC of capacitor Cd and outputs a signal indicating the detected value. Reference voltage generating circuit 21 generates reference voltage VDCr1. Subtracter 22 subtracts terminal-to-terminal voltage VDC of capacitor Cd from reference voltage VDCr1 to obtain deviation ΔVDC=VDCr1−VDC between reference voltage VDCr1 and DC voltage VDC.

DC voltage control circuit 23 calculates current command value Ic for controlling AC input currents I1 to I3 of converter 1 such that deviation ΔVDC=VDCr1−VDC becomes zero. DC voltage control circuit 23 calculates current command value Ic, for example, by performing proportional operation or proportional integral operation of deviation ΔVDC.

Sine wave generating circuit 24 generates three phase sine wave signals having the same phase as three phase AC voltages Vu1, Vv1, and Vw1 from commercial AC power supply 6. Multipliers 25A to 25C multiply the three phase sine wave signals by current command value Ic to generate three phase current command values I1 c to I3 c, respectively.

Subtracter 26A calculates deviation ΔI1=I1 c−I1 between current command value I1 c and AC current I1 detected by current detector CT1. Subtracter 26B calculates deviation ΔI2=I2 c−I2 between current command value I2 c and AC current I2 detected by current detector CT2. Subtracter 26C calculates deviation Δ3=I3 c−I3 between current command value I3 c and AC current I3 detected by current detector CT3.

Current control circuit 27 generates voltage command values V1 a, V2 a, and V3 a such that each of deviations ΔI1, ΔI2, and ΔI3 becomes zero. Current control circuit 27 generates voltage command values V1 a, V2 a, and V3 a, for example, by performing proportional control or proportional integral control of deviations ΔI1, ΔI2, and ΔI3. Voltage detector 28 detects instantaneous values of three phase AC voltages Vu1, Vv1, and Vw1 from commercial AC power supply 6 and outputs signals indicating their detected values.

Adder 29A adds voltage command value Via to AC voltage Vu1 detected by voltage detector 28 to generate voltage command value V1 c. Adder 29B adds voltage command value V2 a to AC voltage Vv1 detected by voltage detector 28 to generate voltage command value V2 c. Adder 29C adds voltage command value V3 a to AC voltage Vw1 detected by voltage detector 28 to generate voltage command value V3 c.

PWM circuit 30 generates PWM control signals ϕ1 to ϕ3 for controlling converter 1, based on voltage command values V1 c to V3 c. OR gate 31 outputs OR signal ϕ31 of switch signal ϕC and power failure detection signal ϕF. When output signal ϕ31 of OR gate 31 is “L” level, gate circuit 32 generates gate signals A1 to A3 and B1 to B3 (FIG. 2) based on PWM control signals ϕ1 to ϕ3.

When output signal ϕ31 of OR gate 31 is “H” level (that is, in the switching period or at the time of a power failure of commercial AC power supply 6), gate circuit 32 brings gate signals A1 to A3 and B1 to B3 to “L” level and turns off IGBTs Q1 to Q6 to stop the operation of converter 1.

The configuration and the control method of bidirectional chopper 2 will now be described. FIG. 10 is a circuit diagram showing a configuration of bidirectional chopper 2. In FIG. 10, bidirectional chopper 2 includes IGBTs Q21 and Q22, diodes D21 and D22, a reactor 35, and a capacitor 36. Bidirectional chopper 2 is controlled by a control unit 37 included in control device 5.

The collector of IGBT Q21 is connected to high voltage-side node 2 a, and the emitter thereof is connected to low voltage-side node 2 c through reactor 35 and connected to the collector of IGBT Q22. The emitter of IGBT Q22 is connected to high voltage-side node 2 b and low voltage-side node 2 d. Diodes D21 and D22 are respectively connected in anti-parallel with IGBTs Q21 and Q22. Capacitor 36 is connected between high voltage-side nodes 2 a and 2 b to stabilize DC voltage VDC between high voltage-side nodes 2 a and 2 b.

IGBT Q21 is controlled by gate signal G1 from control unit 37. When output signal ϕ31 of OR gate 31 (FIG. 9) is “L” level, control unit 37 brings gate signal G1 to “H” level and “L” level at a predetermined frequency, and when signal ϕ31 is “H” level (that is, in the switching period and at the time of a power failure of commercial AC power supply 6), control unit 37 fixes gate signal G1 to “L” level. IGBT Q21 turns on when gate signal G1 is brought to “H” level, and IGBT Q21 turns off when gate signal G1 is brought to “L” level.

When IGBT Q21 is turned on with VDC>VB, current Ib flows through a path from DC positive bus Lp to DC negative bus Ln through IGBT Q21, reactor 35, and battery 9, so that battery 9 is charged and electromagnetic energy is stored into reactor 35.

When IGBT Q21 is turned off, current flows through a path from one terminal (the terminal on the battery 9 side) of reactor 35 to the other terminal of reactor 35 through battery 9 and diode D22, so that battery 9 is charged and electromagnetic energy of reactor 35 is emitted.

The ratio between the period of time in which gate signal G1 is brought to “H” level (pulse width) and one period is called duty ratio. When output signal ϕ31 of OR gate 31 is “L” level, control unit 37 adjusts the duty ratio of gate signal G1 such that terminal-to-terminal voltage VB of battery 9 becomes reference voltage VBr.

IGBT Q22 is controlled by gate signal G2 from control unit 37. When output signal ϕ31 of OR gate 31 (FIG. 9) is “H” level (that is, in the switching period and at the time of a power failure of commercial AC power supply 6), control unit 37 brings gate signal G2 to “H” level and “L” level at a predetermined frequency, and when signal ϕ31 is “L” level, control unit 37 fixes gate signal G2 to “L” level. IGBT Q22 turns on when gate signal G2 is brought to “H” level, and IGBT Q22 turns off when gate signal G22 is brought to “L” level.

When IGBT Q2 is turned on, current flows from the positive electrode of battery 9 to the negative electrode of battery 9 through reactor 35 and IGBT Q22, and electromagnetic energy is stored into reactor 35. When IGBT Q22 is turned off, current flowing from reactor 35 to IGBT Q22 is commutated from reactor 35 to diode D21 and flows to the negative electrode of battery 9 through capacitors 36 and Cd, so that capacitors 36 and Cd are charged and electromagnetic energy of reactor 35 is emitted. When signal ϕ31 is “H” level, control unit 37 adjusts the duty ratio of gate signal G2 such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDC2.

FIGS. 11(A) to 11(H) are time charts showing the operation of control device 5 shown in FIG. 8 to FIG. 10. In FIG. 11, (A) shows the waveform of mode select signal MS, (B) shows the waveform of switch command signal PC, (C) shows the waveform of switch signal ϕC, and (D) shows the waveform of overlap command signal ϕOL.

(E) shows terminal-to-terminal voltage VDC of capacitor Cd, (F) shows the state of switches S1 to S3, (G) shows the state of switches S4 to S6, and (H) shows the state of switches S7 to S9. FIG. 11 shows the operation in a case where the inverter power feed mode is switched to the bypass power feed mode.

At time t0, the inverter power feed mode is executed, and all of mode select signal MS, switch command signal PC, switch signal ϕC, and overlap command signal ϕOL are brought to “L” level. Terminal-to-terminal voltage VDC of capacitor Cd is brought to reference voltage VDCr1, switches S1 to S6 are turned on, and switches S7 to S9 are turned off.

When the bypass power feed mode is selected using operation unit 4 at a certain time t1, mode select signal MS is raised from “L” level to “H” level, and switch command signal PC is raised to “H” level by signal generating circuit 11 for a predetermined period of time. In response to the rising edge of switch command signal PC, timer 12 (FIG. 8) successively measures first time T1, second time T2, and third time T3 and generates switch signal ϕC and overlap command signal ϕOL based on the time measurement result.

Switch signal ϕC is brought to “H” level from the rising edge of switch command signal PC (time t1) to third time T3 (time t4). Overlap command signal ϕOL is brought to “H” level from first time T1 (time t2) to second time T2 (time t3).

When switch signal ϕC is raised from “L” level to “H” level (time t1), switches S1 to S3 (FIG. 1) are turned off, the operation of converter 1 is stopped by control unit 16 (FIG. 8, FIG. 9), and bidirectional chopper 2 is controlled by control unit 37 (FIG. 10) such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2.

In the switching period in which switches S1 to S3 are turned off, lap command signal ϕOL is brought to “H” level, and the lap power feed mode is executed. When lap command signal ϕOL is raised to “H” level (time t2), switches S7 to S9 are turned on. At this point of time, since switches S1 to S3 are off, circulating current IC (FIG. 6, FIG. 7) does not flow. When lap command signal ϕOL is lowered to “L” level (time t3), switches S4 to S6 are turned off, and the lap power feed mode ends.

When switch signal ϕC is lowered to “L” level (time t4), switches S1 to S3 are turned on, the operation of converter 1 is resumed, charging of battery 9 by bidirectional chopper 2 is resumed, and the switching from the inverter power feed mode to the bypass power feed mode is completed. Terminal-to-terminal voltage VDC of capacitor Cd is raised from reference voltage VDCr2 to reference voltage VDCr1 by converter 1. At this point of time, since reference voltage VDCr1 is set to a voltage slightly higher than reference voltage VDCr2, terminal-to-terminal voltage VDC of capacitor Cd can be quickly returned to reference voltage VDCr1.

FIGS. 12(A) to 12(H) are other time charts showing the operation of control device 5 shown in FIG. 8 to FIG. 10, in comparison with FIGS. 11(A) to 11(H). FIG. 12(A) to 12(H) show the operation in a case where the bypass power feed mode is switched to the inverter power feed mode.

At time t0, the bypass power feed mode is executed, mode select signal MS is brought to “H” level, and all of switch command signal PC, switch signal ϕC, and overlap command signal ϕOL are set to “L” level. Terminal-to-terminal voltage VDC of capacitor Cd is brought to reference voltage VDCr1 by converter 1, switches S1 to S3 and S7 to S9 are turned on, and switches S4 to S6 are turned off.

When the inverter power feed mode is selected using operation unit 4 at a certain time t1, mode select signal MS is lowered from “H” level to “L” level, and switch command signal PC is raised to “H” level by signal generating circuit 11 for a predetermined period of time. In response to the rising edge of switch command signal PC, timer 12 (FIG. 8) successively measures first time T1, second time T2, and third time T3 and generates switch signal ϕC and overlap command signal ϕOL based on the time measurement result.

Switch signal ϕC is brought to “H” level from the rising edge of switch command signal PC (time t1) to third time T3 (time t4). Overlap command signal ϕOL is set to “H” level from first time T1 (time t2) to second time T2 (time t3).

When switch signal ϕC is raised from “L” level to “H” level (time t1), the operation of converter 1 is stopped by control unit 16 (FIG. 8, FIG. 9), and bidirectional chopper 2 is controlled by control unit 37 (FIG. 10) such that terminal-to-terminal voltage VDC of capacitor Cd becomes reference voltage VDCr2.

In the period in which switches S1 to S3 are turned of, lap command signal ϕOL is brought to “H” level, and the lap power feed mode is executed. When lap command signal ϕOL is raised to “H” level (time t2), switches S4 to S6 are turned on. At this point of time, since switches S1 to S3 are off, circulating current IC (FIG. 6, FIG. 7) does not flow. When lap command signal ϕOL is lowered to “L” level (time t3), switches S7 to S9 are turned off, and the lap power feed mode ends.

When switch signal ϕC is lowered to “L” level (time t4), switches S1 to S3 are turned on, the operation of converter 1 is resumed, charging of battery 9 by bidirectional chopper 2 is resumed, and the switching from the bypass power feed mode to the inverter power feed mode is completed. Terminal-to-terminal voltage VDC of capacitor Cd is raised from reference voltage VDCr2 to reference voltage VDCr1 by converter 1. At this point of time, since reference voltage VDCr1 is set to a voltage slightly higher than reference voltage VDCr2, terminal-to-terminal voltage VDC of capacitor Cd can be quickly returned to reference voltage VDCr1.

As described above, in the present first embodiment, in the switching period in which the inverter power feed mode and the bypass power feed mode are switched, switches S1 to S3 are turned off to electrically isolate converter 1 from commercial AC power supply 6. Therefore, even when both of neutral point terminal 6 d of commercial AC power supply 6 and neutral point terminal 7 d of bypass AC power supply 7 are grounded, flowing of circulating current IC through a path including capacitor Cd can be prevented.

Second Embodiment

FIG. 13 is a circuit block diagram showing the main part of an uninterruptible power supply apparatus according to a second embodiment of the present invention, in comparison with FIG. 8. Referring to FIG. 13, this uninterruptible power supply apparatus differs from the first embodiment in that timer 12 and control units 14 and 15 are replaced by a control unit 40 and auxiliary switches Sa, Sb, and Sc and state detectors 44 to 46 are added.

Switches S1 to S3 and auxiliary switch Sa constitute an electromagnetic contact 41. Auxiliary switch Sa is interlocked with switches S1 to S3. For example, when switches S1 to S3 turn on, switch Sa also turns on. Conversely, when switches S1 to S3 turn off, switch Sa may turn on. Switch Sa is connected to state detector 44. State detector 44 detects whether switch Sa turns on or turns off (that is, whether switches S1 to S3 turn on or turn off) and outputs signal ϕ44 indicating the detection result to control unit 40.

Switches S4 to S6 and auxiliary switch Sb constitute an electromagnetic contact 42. Auxiliary switch Sb is interlocked with switches S4 to S6. For example, when switches S4 to S6 turn on, switch Sb also turns on. Conversely, when switches S4 to S6 turn off, switch Sb may turn on. Switch Sb is connected to state detector 45. State detector 45 detects whether switch Sb turns on or turns off (that is, whether switches S4 to S6 turn on or turn off) and outputs signal ϕ45 indicating the detection result to control unit 40.

Switches S7 to S9 and auxiliary switch Sc constitute an electromagnetic contact 43. Auxiliary switch Sc is interlocked with switches S7 to S9. For example, when switches S7 to S9 turn on, switch Sc also turns on. Conversely, when switches S7 to S9 turn off, switch Sc may turn on. Switch Sc is connected to state detector 46. State detector 46 detects whether switch Sc turns on or turns off (that is, whether switches S7 to S9 turn on or turn off) and outputs signal 46 indicating the detection result to control unit 40.

Control unit 40 controls electromagnetic contacts 41 to 43 and outputs switch signal ϕC, based on output signal PS of signal generating circuit 11, output signals ϕ44 to ϕ46 of state detectors 44 to 46, and mode select signal MS. Control unit 40 turns off electromagnetic contact 41 when output signal ϕF of power failure detector 13 is “H” level that is the active level.

FIGS. 14(A) to 14(F) are time charts showing the operation of control unit 40 shown in FIG. 13. In FIG. 14, (A) shows the waveform of mode select signal MS, (B) shows the waveform of switch command signal PC, (C) shows the waveform of switch signal ϕC, (D) shows the state of switches S1 to S3 and Sa, (E) shows the state of switches S4 to S6 and Sb, and (F) shows the state of switches S7 to S9 and Sc. FIG. 14 shows the operation in a case where the inverter power feed mode is switched to the bypass power feed mode.

At time t0, the inverter power feed mode is executed, and all of mode select signal MS, switch command signal PC, and switch signal ϕC are set to “L” level. Switches S1 to S3 and Sa and switches S4 to S6 and Sb are turned on, and switches S7 to S9 and Sc are turned off.

When the bypass power feed mode is selected using operation unit 4 at a certain time t1, mode select signal MS is raised from “L” level to “H” level, and switch command signal PC is raised to “H” level by signal generating circuit 11 for a predetermined period of time. In response to switch command signal PC being raised to “H” level, control unit 40 raises switch signal ϕC to “H” level and turns off switches S1 to S3 and Sa (time t2).

In response to auxiliary switch Sa being turned off, control unit 40 turns on switches S7 to S9 and Sc to execute the lap power feed mode (time t3). At this point of time, since switches S1 to S3 are off, circulating current IC (FIG. 6, FIG. 7) does not flow.

In response to auxiliary switch Sc being turned off, control unit 40 turns off switches S4 to S6 and Sb to terminate the lap power feed mode (time t4). In response to auxiliary switch Sb being turned of, control unit 40 turns on switches S1 to S3 and Sa and lowers switch signal ϕC to “L” level (time t5). The switching from the inverter power feed mode to the bypass power feed mode then ends.

FIGS. 15(A) to 15(F) are other time charts showing the operation of control unit 40 shown in FIG. 13. In FIG. 15, (A) shows the waveform of mode select signal MS, (B) shows the waveform of switch command signal PC, (C) shows the waveform of switch signal ϕC, (D) shows the state of switches S1 to S3 and Sa, (E) shows the state of switches S4 to S6 and Sb, and (F) shows the state of switches S7 to S9 and Sc. FIG. 15 shows the operation in a case where the bypass power feed mode is switched to the inverter power feed mode.

At time t0, the bypass power feed mode is executed, mode select signal MS is brought to “H” level, and both of switch command signal PC and switch signal ϕC are brought to “L” level. Switches S1 to S3 and Sa and switches S7 to S9 and Sc are turned on, and switches S4 to S6 and Sb are turned off.

When the inverter power feed mode is selected using operation unit 4 at a certain time t1, mode select signal MS is lowered from “H” level to “L” level, and switch command signal PC is raised to “H” level by signal generating circuit 11 for a predetermined period of time. In response to switch command signal PC being raised to “H” level, control unit 40 raises switch signal ϕC to “H” level and turns off switches S1 to S3 and Sa (time t2).

In response to auxiliary switch Sa being turned off, control unit 40 turns on switches S4 to S6 and Sb to execute the lap power feed mode (time t3). At this point of time, since switches S1 to S3 are off, circulating current IC (FIG. 6, FIG. 7) does not flow.

In response to auxiliary switch Sb being turned off, control unit 40 turns off switches S7 to S9 and Sc to terminate the lap power feed mode (time t4). In response to auxiliary switch Sc being turned off, control unit 40 turns on switches S1 to S3 and Sa and lowers switch signal ϕC to “L” level (time t5). The switching from the bypass power feed mode to the inverter power feed mode then ends.

The other configuration and operation is similar to the first embodiment and a description thereof is not repeated. The present second embodiment achieves the same effect as the first embodiment.

The embodiments disclosed here should be understood as being illustrative in all respects and should not be construed as being limiting. The present invention is shown not in the foregoing description but in the claims, and it is intended that all modifications that come within the meaning and range of equivalence to the claims are embraced here.

REFERENCE SIGNS LIST

S1 to S9 switch, C1 to C6, Cd, 36 capacitor, L1 to L6, 35 reactor, CT1 to CT6 current detector, 1 converter, Lp DC positive bus, Ln DC negative bus, 2 bidirectional chopper, 3 inverter, 4 operation unit, 5 control device, 6 commercial AC power supply, 6 d, 7 d neutral point terminal, 6U, 6V, 6W, 7U, 7V, 7W AC power supply, 7 bypass AC power supply, 8 load, 9 battery, Q1 to Q6, Q11 to Q16, Q21, Q22 IGBT, D1 to D6, D1 to D16, D21, D22 diode, 11 signal generating circuit, 12 timer, 13 power failure detector, 14 to 16, 37, 40 control unit, 20, 28 voltage detector, 21 reference voltage generating circuit, 22, 26A to 26C subtracter, 23 DC voltage control circuit, 24 sine wave generating circuit, 25A to 25C multiplier, 27 current control circuit, 29A to 29C adder, 30 PWM circuit, 31 gate circuit, Sa, Sb, Sc auxiliary switch, 41 to 43 electromagnetic contact, 44 to 46 state detector. 

1. An uninterruptible power supply apparatus comprising: a forward converter that converts three-phase AC voltage to DC voltage; a capacitor that smoothes DC output voltage of the forward converter; a reverse converter that converts terminal-to-terminal voltage of the capacitor to three-phase AC voltage; a bidirectional chopper that exchanges DC power between the capacitor and a power storage device; a first switch disposed corresponding to each phase of three-phase AC voltage supplied from a first AC power supply, the first switch having one terminal receiving AC voltage of a corresponding phase and another terminal connected to the forward converter; a second switch disposed corresponding to each phase of three-phase AC voltage output from the reverse converter, the second switch having one terminal receiving AC voltage of a corresponding phase and another terminal connected to a load; a third switch disposed corresponding to each phase of three-phase AC voltage supplied from a second AC power supply, the third switch having one terminal receiving AC voltage of a corresponding phase and another terminal connected to the load; a first control unit; and a second control unit that controls the reverse converter in synchronization with three-phase AC voltage supplied from the second AC power supply, wherein the first control unit turns on the first and second switches and turns off the third switch in a first mode of supplying three-phase AC voltage from the reverse converter to the load, turns on the first and third switches and turns off the second switch in a second mode of supplying three-phase AC voltage from the second AC power supply to the load, and turns off the first switch and turns on the second and third switches and executes a third mode of supplying three-phase AC voltage from both of the reverse converter and the second AC power supply to the load in a switching period in which one mode of the first and second modes is switched to another mode, and in the first and second modes, the second control unit controls the forward converter such that terminal-to-terminal voltage of the capacitor becomes a first reference voltage and controls the bidirectional chopper such that terminal-to-terminal voltage of the power storage device becomes a second reference voltage, and in the switching period, the second control unit stops operation of the forward converter, and controls the bidirectional chopper such that terminal-to-terminal voltage of the capacitor becomes a third reference voltage.
 2. The uninterruptible power supply apparatus according to claim 1, wherein in the first mode, the second control unit controls the forward converter such that terminal-to-terminal voltage of the capacitor becomes the first reference voltage and controls the bidirectional chopper such that terminal-to-terminal voltage of the power storage device becomes the second reference voltage, in a sound state of the first AC power supply, and controls the bidirectional chopper such that terminal-to-terminal voltage of the capacitor becomes the third reference voltage, at a time of a power failure of the first AC power supply.
 3. The uninterruptible power supply apparatus according to claim 1, wherein the third reference voltage is lower than the first reference voltage.
 4. The uninterruptible power supply apparatus according to claim 1, further comprising: a selector that selects a desired mode of the first and second modes; a signal generating circuit that outputs a switch command signal in response to a mode selected by the selector being changed from one mode to another mode; and a timer that successively measures a first time, a second time, and a third time, in response to the switch command signal, wherein in the switching period, the first control unit turns off the first switch in response to the switch command signal, turns on a switch corresponding to the other mode of the second and third switches, in response to the first time being measured by the timer, turns off a switch corresponding to the one mode of the second and third switches, in response to the second time being measured by the timer, and turns on the first switch in response to the third time being measured by the timer.
 5. The uninterruptible power supply apparatus according to claim 1, further comprising: a selector that selects a desired mode of the first and second modes; a signal generating circuit that outputs a switch command signal in response to a mode selected by the selector being changed from one mode to another mode; first to third auxiliary switches interlocked with the first to third switches, respectively; and first to third state detectors that detect whether the first to third switches turn on or turn off, respectively, based on a conducting state of the first to third auxiliary switches, wherein in the switching period, the first control unit turns off the first switch in response to the switch command signal, turns on a switch corresponding to the other mode of the second and third switches, in response to turning-off of the first switch being detected by the first state detector, turns off a switch corresponding to the one mode of the second and third switches, in response to turning-on of a switch corresponding to the other mode being detected by the second or third state detector, and turns on the first switch in response to turning-off of a switch corresponding to the one mode being detected by the second or third state detector.
 6. The uninterruptible power supply apparatus according to claim 1, wherein each of the first and second AC power supplies includes a three-phase AC power supply star-connected to a neutral point, and both of neutral points of the first and second AC power supplies are grounded.
 7. The uninterruptible power supply apparatus according to claim 1, wherein the first AC power supply is a commercial AC power supply, and the second AC power supply is a power generator. 